Programming iOS 4

Subview and Superview

Once upon a time, and not so very long ago, a view owned precisely its rectangular area. No part of any view that was not a subview of this view could appear inside it, because when this view redrew its rectangle, it would erase the overlapping portion of the other view. No part of any subview of this view could appear outside it, because the view took responsibility for its own rectangle and no more.

Those rules, however, were gradually relaxed, and starting in Mac OS X 10.5 Apple introduced an entirely new architecture for view drawing that lifted those restrictions completely. iOS view drawing is based on this revised architecture. So now some or all of a subview can appear outside its superview, and a view can overlap another view and be drawn partially or totally in front of it without being its subview.

So, for example, Figure 14-1 shows three overlapping views. All three views have a background color, so each is completely represented by a colored rectangle. You have no way of knowing, from this visual representation, how the views are related within the view hierarchy. In actual fact, the view in the middle (horizontally) is a sibling view of the view on the left (they are both direct subviews of the window), and the view on the right is a subview of the middle view.

Figure 14-1. Overlapping views

When views are created in the nib, you can examine the view ...